Registered: 1388591029 Posts: 2,907
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The irradiation at any given spot is NOT static. If it were you could simply call up the weather bureau. As it is, all they can give you is an average of several years' observations. That's fine if you want to take several more years to evaluate your collector, but none of us have that kind of time. The only other way I know is to measure it in real time.
Also, the standard way to measure irradiation is normal to the earth's surface which is pretty much horizontal, but who mounts their collectors horizontally? No one I know of, willingly. We mount them vertically or angled toward the sun. So it's possible to intercept MORE energy than is hitting a horizontal surface, particularly in the winter when the sun is low.
BTUs are good, but if I want more of them all I have to do is build a bigger collector, not necessarily a better collector. That's why we have side by side testing, but it's not always possible. So some of us try to calculate efficiency and develop our collectors that way. __________________ Solar is like the wind. It may be free, but putting it to work isn't! Willie, Tampa Bay